Method and apparatus for identifying an islanding condition

ABSTRACT

A method and apparatus for identifying an islanding condition. In one embodiment, the method comprises altering a phase error response within a phase locked loop (PLL), and determining whether the islanding condition exists based on the altered phase error response.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation of U.S. patent application Ser.No. 12/218,752, filed Jul. 16, 2008, now U.S. Pat. No. 7,899,632, whichclaims benefit of U.S. provisional patent application Ser. No.60/959,644, filed Jul. 16, 2007. Each of the aforementioned patentapplications is herein incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present disclosure generally relate to a method andapparatus for anti-islanding of distributed power generation systems.

2. Description of the Related Art

Solar panels have historically been deployed in mostly remoteapplications, such as remote cabins in the wilderness or satellites,where commercial power was not available. Due to the high cost ofinstallation, solar panels were not an economical choice for generatingpower unless no other power options were available. However, theworldwide growth of energy demand is leading to a durable increase inenergy cost. In addition, it is now well established that the fossilenergy reserves currently being used to generate electricity are rapidlybeing depleted. These growing impediments to conventional commercialpower generation make solar panels a more attractive option to pursue.

Solar panels, or photovoltaic (PV) modules, convert energy from sunlightreceived into direct current (DC). The PV modules cannot store theelectrical energy they produce, so the energy must either be dispersedto an energy storage system, such as a battery or pumpedhydroelectricity storage, or dispersed by a load. One option to use theenergy produced is to employ inverters to convert the DC current into analternating current (AC) and couple the AC current to the commercialpower grid. The power produced by such a distributed generation (DG)system can then be sold to the commercial power company.

Under some conditions, a grid-connected DG system may becomedisconnected from the utility grid, resulting in a potentially dangerouscondition known as “islanding”. During islanding, the utility cannotcontrol voltage and frequency in the DG system island, creating thepossibility of damage to customer equipment coupled to the island.Additionally, an island may create a hazard for utility line workers orthe general public by causing a line to remain energized that is assumedto be disconnected from all energy sources. In order to mitigate thepotential hazards of islanding, the IEEE standard 929-2000 requiresinverters in a DG system detect the loss of the utility grid and shutdown the inverter within two seconds. As such, all commerciallyavailable inverters, including each micro-inverter of a micro-inverterarray, must be equipped with an inverter-based anti-islandingcapability. Current techniques employed to meet such a standard requiresubstantial power, thus reducing the efficiency of the inverter.

Therefore, there is a need in the art for a method and apparatus forfast detection of islanding in a grid-connected inverter.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to a method andapparatus for identifying an islanding condition. In one embodiment, themethod comprises altering a phase error response within a phase lockedloop (PLL), and determining whether the islanding condition exists basedon the altered phase error response.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a system for distributed generation inaccordance with one or more embodiments of the present invention;

FIG. 2 is a block diagram of a control module in accordance with one ormore embodiments of the present invention;

FIG. 3 is a block diagram of a micro-inverter in accordance with one ormore embodiments of the present invention;

FIG. 4 is a block diagram of a Digital Phase Locked Loop in accordancewith one or more embodiments of the present invention.

FIG. 5 is a block diagram of a phase error signature monitor inaccordance with one or more embodiments of the present invention;

FIG. 6 is a graphical diagram of a phase error response in the presenceof a connected grid in accordance with one or more embodiments of thepresent invention;

FIG. 7 is a graphical diagram of a phase error response in the absenceof a connected grid in accordance with one or more embodiments of thepresent invention;

FIG. 8 is a flow diagram of a method for detecting an islanding state ina grid-connected inverter in accordance with one or more embodiments ofthe present invention; and

FIG. 9 is a flow diagram of a method for synchronizing phase shiftinjection in a plurality of micro-inverters in accordance with one ormore embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a system 100 for distributed generation(DG) in accordance with one or more embodiments of the presentinvention. This diagram only portrays one variation of the myriad ofpossible system configurations. The present invention can function in avariety of distributed power generation environments and systems.

The system 100 comprises a plurality of micro-inverters 102 ₁, 102 ₂ . .. 102 _(n), collectively referred to as micro-inverters 102, a pluralityof PV modules 104 ₁, 104 ₂ . . . 104 _(n), collectively referred to asPV modules 104, an AC bus 106, a load center 108, and an array controlmodule 110.

Each micro-inverter 102 ₁, 102 ₂ . . . 102 _(n) is coupled to a PVmodule 104 ₁, 104 ₂ . . . 104 _(n), respectively. The micro-inverters102 are further coupled to the AC bus 106, which in turn is coupled tothe load center 108. The load center 108 houses connections betweenincoming power lines from a commercial power grid distribution systemand the AC bus 106. The micro-inverters 102 convert DC power generatedby the PV modules 104 into AC power, and meter out AC current that isin-phase with the AC commercial power grid voltage. The system 100couples the generated AC power to the commercial power grid via the loadcenter 108.

A control module 110 is coupled to the AC bus 106. The control module110 is capable of issuing command and control signals to themicro-inverters 102 in order to control the functionality of themicro-inverters 102.

FIG. 2 is a block diagram of a control module 110 in accordance with oneor more embodiments of the present invention. The control module 110comprises a transceiver 202 coupled to at least one central processingunit (CPU) 204. The CPU is additionally coupled to support circuits 206,and a memory 208. The CPU 204 may comprise one or more conventionallyavailable microprocessors. Alternatively, the CPU 204 may include one ormore application specific integrated circuits (ASIC). The supportcircuits 206 are well known circuits used to promote functionality ofthe central processing unit. Such circuits include, but are not limitedto, a cache, power supplies, clock circuits, buses, network cards,input/output (I/O) circuits, and the like.

The memory 208 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 208 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory208 generally stores the operating system 214 of the control module 110.The operating system 214 may be one of a number of commerciallyavailable operating systems such as, but not limited to, SOLARIS fromSUN Microsystems, Inc., AIX from IBM Inc., HP-UX from Hewlett PackardCorporation, LINUX from Red Hat Software, Windows 2000 from MicrosoftCorporation, and the like.

The memory 208 may store various forms of application software, such asmicro-inverter control software 210. The transceiver 202 communicablycouples the control module 110 to the micro-inverters 102 to facilitatecommand and control of the micro-inverters 102. The transceiver 202 mayutilize wireless or wired communication techniques for suchcommunication. In one embodiment, the micro-inverter control software210 synchronizes the anti-islanding hardware and/or software of themicro-inverters 102, which is further described below.

FIG. 3 is a block diagram of a micro-inverter 102 in accordance with oneor more embodiments of the present invention. The micro-inverter 102comprises a power conversion circuit 302, a conversion control module303, a digital phase-locked loop (DPLL) 304, a phase shift generator306, and a phase error signature monitor 308. The DPLL 304 is coupled tothe conversion control module 303, the phase shift generator 306, andthe phase error signature monitor 308. The conversion control module 303is further coupled to the phase error signature monitor 308 and thepower conversion circuit 302. The power conversion circuit 302 iscoupled to the PV module 104 and acts to convert a DC current from thePV module 104 to an AC current; the conversion control module 303provides operative control of the power conversion circuit 302. The DPLL304 receives a grid voltage reference signal and locks to the frequencyof the grid voltage; additionally the DPLL receives a nominal cycleinput that provides the nominal period of the grid system in order toprevent the DPLL 304 from changing when the nominal grid frequencychanges. The DPLL 304 provides an input to the conversion control module303 that drives the power conversion circuit 302 to inject the generatedAC output current in phase with the grid as required by the relevantstandards.

The phase shift injector 306 injects a small phase shift through theDPLL 304. In one embodiment, where the connected grid operates at afrequency of 60 Hz, a phase shift of magnitude 50 microseconds over aperiod of one cycle (i.e., 16.7 milliseconds) is injected at 0.5 secondintervals; such an injected phase shift represents a phase shift of onedegree and causes an insignificant distortion to the current injectedinto the grid and/or load. Alternative embodiments may utilize differentphase shift magnitudes, durations, and/or injection intervals. While themicro-inverter 102 remains connected to the utility grid, the DPLL 304produces a certain phase error response as a result of the injectedphase shift. If the micro-inverter 102 becomes disconnected from thegrid, the DPLL 304 produces a different phase error response as a resultof the injected phase shift. Such phase error responses are shown inFIGS. 6 and 7 and further described below. The phase error signaturemonitor 308 monitors the phase error response of the DPLL 304 todetermine when the micro-inverter 102 is no longer connected to thegrid, creating an island. In the event of an island, the phase errorsignature monitor 308 provides a deactivation signal to the conversioncontrol module 303 to shut down the power conversion circuit 302.

FIG. 4 is a block diagram of a Digital Phase Locked Loop (DPLL) 304 inaccordance with one or more embodiments of the present invention. Such aDPLL 304 can be implemented in hardware, software, or a combination ofhardware and software. The DPLL 304 comprises a phase detector 402, anadder 404, a Proportional Integral Derivative (PID) controller 406, anadder 408, and a numerically controlled oscillator (NCO) 410. The phasedetector 402 receives a first input from a reference signal of the gridvoltage frequency as described above; the output of the NCO 410 iscoupled to the phase detector 402 and provides a second input. Theoutput from the NCO 410 and the reference signal of the grid voltagefrequency are compared by the phase detector 402 to produce a resultingphase error. This phase error output of the phase detector 402 iscoupled to the adder 404 along with the output of the phase shiftgenerator 306. The phase shift generator 306 injects a phase shift asdescribed above. The resulting output of the adder 404 is coupled to thePID controller 406. In one embodiment, the PID controller 406 acts asthe loop filter for the DPLL 304; alternative embodiments may compriseother forms of loop filter implementations. The PID controller 406output comprises a phase error response that is coupled to the phaseerror signature monitor 308. Additionally, the PID controller 406 outputis coupled to the adder 408 along with a nominal cycle time input. Inone embodiment, the nominal cycle time is 1/60 seconds. The resultingoutput of the adder 408 is coupled to the NCO 410. The NCO 410 output,in addition to being coupled to the phase detector 402, is coupled tothe conversion control module 303 to drive the generated AC currentinjection into the grid as described above.

FIG. 5 is a block diagram of a phase error signature monitor 308 inaccordance with one or more embodiments of the present invention. Such aphase error signature monitor 308 can be implemented in hardware,software, or a combination of hardware and software. The phase errorsignature monitor 308 comprises a resettable integrator 502 coupled to asampler 504, and a resettable integrator 509 coupled to a sampler 510.The outputs of the samplers 504 and 510 are coupled to a subtractor 512.The output of the subtractor 512 is coupled to an input of a comparator506. A reference threshold input is coupled to a second input of thecomparator 506. The output of the comparator 506 is coupled to anislanding decision controller 508.

The phase error response at the output of the PID controller 406 iscoupled to the resettable integrators 502 and 509. During a sampleperiod, the resettable integrators 502 and 509 are both reset, and theresettable integrator 509 integrates a baseline phase error responseover a baseline period. In one embodiment, the sample period is 0.5seconds and the baseline period is seven consecutive grid cycles (i.e.,116.667 milliseconds for a 60 Hz grid voltage). Following the baselineperiod, the sampler 510 samples the output of the integrator 509 andprovides the resulting baseline integrated phase error response value tothe subtractor 512. Also following the baseline period, a phase shift isinjected; in one embodiment, the phase shift is injected during the gridcycle immediately following the baseline period. The resettableintegrator 502 integrates the phase error response resulting from theinjected phase shift over an integration period. In one embodiment, theintegration period is seven consecutive grid voltage cycles immediatelyfollowing the phase shift injection (e.g., 116.667 milliseconds for a 60Hz grid voltage). After the integration period, the sampler 504 samplesthe output of the integrator 502 and provides the resulting integratedphase error response value to the subtractor 512. The subtractor 512subtracts the baseline integrated phase error response value from theintegrated phase error response value resulting from the phase shiftinjection; the output of the subtractor 512 is provided to an input ofthe comparator 506. In alternative embodiments where the grid voltagefrequency remains stable, the baseline integrated phase error responseis not required, and the phase error signature monitor 308 can thusly beimplemented without the resettable integrator 509, the sampler 510, andthe subtractor 512. In such alternative embodiments, the output of thesampler 504 is coupled to the input of the comparator 506.

The comparator 506 compares the resulting difference to a referencethreshold. If the threshold is satisfied, the sample period containingthe injected phase shift is considered indicative of a potential griddisconnection. The output of the comparator 506 is coupled to theislanding decision controller 508. The islanding decision controller 508determines whether “n” out of the “p” most recent sample periodsindicate a potential grid disconnection; if this condition is satisfied,an islanding state is declared and the islanding decision controller 508issues a control signal to the conversion control module 303 to shutdown the power conversion circuit 302.

FIG. 6 is a graphical diagram of a phase error response 600 in thepresence of a connected grid in accordance with one or more embodimentsof the present invention. In the presence of a connected grid, the phaseshift injected by the phase shift generator 306 causes an insignificantdistortion to the current injected into the grid and produces no voltageimpact on the grid. Thus, the grid voltage reference signal to the phasedetector 402 does not change as a result of the injected phase shift,and the DPLL 304 compensates for the injected phase error such that thebipolar phase error response 600 is generated. Integrating the bipolarphase error response 600 over time results in a value of about zero.Thus, the reference threshold input to the comparator 506 can be setsuch it is not exceeded by the difference between the baselineintegrated phase error response and the integrated phase error responseresulting from the injected phase shift, and the output of thecomparator 506 indicates a continued grid connection.

FIG. 7 is a graphical diagram of a phase error response 700 in theabsence of a connected grid in accordance with one or more embodimentsof the present invention. When the grid becomes disconnected from themicro-inverter 102, the grid voltage reference signal is no longerprovided to the phase detector 402 and is replaced by a reference signalthat is a product of the inverter current and the impedance of a loadcoupled to the inverter. As a result, the DPLL 304 does not providecompensation for the injected phase error, resulting in the unipolarphase error response 700. Integrating the phase error response 700 overtime results in an increasing integrated phase error response value. Thedifference between the baseline integrated phase error response and theincreasing integrated phase error response resulting from the injectedphase shift will at some point exceed the reference threshold input tothe comparator 506, generating an output of the comparator 506indicating a potential grid disconnection. The reference threshold canbe set such that the loss of the grid connection is rapidly detected.

FIG. 8 is a flow diagram of a method 800 for detecting an islandingstate in a grid-connected inverter in accordance with one or moreembodiments of the present invention. The method 800 begins at step 802and proceeds to step 803. At step 803, at the start of a sample period,a baseline phase error response of the DPLL of an inverter isaccumulated over a baseline period. In one embodiment, a sample periodof 0.5 seconds and a baseline period of 116.667 milliseconds (i.e.,seven cycles for a 60 Hz grid voltage) are utilized. The method proceedsto step 804. At step 804, a small phase shift is injected through theDPLL of the inverter such that the phase shift causes an insignificantdistortion to the current injected into the grid. In one embodiment, thephase shift has a magnitude of 50 microseconds over a duration of 16.7milliseconds (i.e., one cycle for a 60 Hz grid voltage). In alternativeembodiments, the phase shift is injected through the DPLL of amicro-inverter.

The method 800 proceeds to step 806. At step 806, a phase error responseof the DPLL resulting from the injected phase shift is accumulated overan integration period; in one embodiment, an integration period of116.667 milliseconds (i.e., seven cycles for a 60 Hz grid voltage) isutilized. At step 807, the accumulated baseline phase error response issubtracted from the accumulated phase error response resulting from theinjected phase shift, and, at step 808, the resulting difference iscompared to a threshold. If the resulting difference does not satisfythe threshold, the method 800 proceeds to step 810. If the currentsample period has not elapsed, the method 800 waits at step 810; if thecurrent sample period has elapsed, the method 800 returns to step 803.

If the resulting difference satisfies the threshold at step 808, themethod 800 proceeds to step 812. At step 812, the current sample periodis flagged as indicating a potential grid disconnection. At step 814,the method 800 determines whether “n” potential grid disconnections haveoccurred within the “p” most recent consecutive sample periods; in oneembodiment, the method 800 determines whether two potential griddisconnections have occurred within the three most recent consecutivesample periods. If the n-out-of-p potential grid disconnections have notoccurred, the method 800 proceeds to step 810. If the n-out-of-ppotential grid disconnections have occurred, the method 800 proceeds tostep 816, where an islanding state is declared and the inverter is shutdown. The method 800 then ends at step 818.

FIG. 9 is a flow diagram of a method 900 for synchronizing phase shiftinjection in a plurality of micro-inverters in accordance with one ormore embodiments of the present invention. The method 900 begins at step902 and proceeds to step 904. At step 904, a control module coupled to aplurality of micro-inverters, such as the control module 110 coupled tomicro-inverters 102 via the AC bus 106, broadcasts a message to theplurality of micro-inverters. In one embodiment, the message comprises amillisecond timestamp. The method proceeds to step 906. At step 906,each micro-inverter of the plurality of micro-inverters simultaneouslyinjects a phase shift through its DPLL, such as in the method 800described above. The method 900 then ends at step 908.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. Apparatus for identifying an islanding condition, comprising: a phaseshift generator for altering a phase error response within a phaselocked loop (PLL); and a phase error signature monitor for determiningwhether the islanding condition exists based on the altered phase errorresponse, wherein the phase error signature monitor (i) compares adifference between a baseline phase error response of the PLL and thealtered phase error response to a threshold, and (ii) determines whetherthe islanding condition exists based on whether the threshold issatisfied.
 2. The apparatus of claim 1, wherein the phase shiftgenerator injects a phase shift into the PLL to generate the alteredphase error response.
 3. The apparatus of claim 1, wherein the baselinephase error response is accumulated over a first period and the alteredphase error response is accumulated over a second period.
 4. Theapparatus of claim 3, wherein the first period and the second periodoccur within a sample period, and the sample period is identified aspotentially indicative of the islanding condition when the threshold issatisfied.
 5. The apparatus of claim 4, wherein the phase errorsignature monitor declares the islanding condition when a first numberof sample periods within a consecutive second number of sample periodsare identified as potentially indicative of the islanding condition. 6.The apparatus of claim 5, wherein the phase error signature monitorinhibits power production in an inverter when the islanding condition isdeclared.
 7. A method for identifying an islanding condition,comprising: altering, by a phase shift generator, a phase error responsewithin a phase locked loop (PLL); and determining, by a phase errorsignature monitor, whether the islanding condition exists based on thealtered phase error response, wherein determining whether the islandingcondition exists comprises (i) comparing a difference between a baselinephase error response of the PLL and the altered phase error response toa threshold, and (ii) determining whether the threshold is satisfied. 8.The method of claim 7, wherein altering the phase error responsecomprises injecting a phase shift into the PLL.
 9. The method of claim7, wherein the baseline phase error response is accumulated over a firstperiod and the altered phase error response is accumulated over a secondperiod.
 10. The method of claim 9, wherein the first period and thesecond period occur within a sample period, and the sample period isidentified as potentially indicative of the islanding condition when thethreshold is satisfied.
 11. The method of claim 10, wherein theislanding condition is declared when a first number of sample periodswithin a consecutive second number of sample periods are identified aspotentially indicative of the islanding condition.
 12. The method ofclaim 11, further comprising inhibiting power production in an inverterwhen the islanding condition is declared.
 13. A system for determiningan islanding state of a distributed power generator, comprising: aplurality of inverters, wherein each inverter of the plurality ofinverters comprises a phase locked loop (PLL), a phase shift generatorfor altering a phase error response within the PLL, and a phase errorsignature monitor for determining whether the islanding condition existsbased on the altered phase error response, wherein the phase errorsignature monitor (i) compares a difference between a baseline phaseerror response of the PLL and the altered phase error response to athreshold, and (ii) determines whether the islanding condition existsbased on whether the threshold is satisfied; and a controller, coupledto the plurality of inverters, for causing each of the phase shiftgenerators to alter the phase error response simultaneously.
 14. Thesystem of claim 13, wherein the phase shift generator injects a phaseshift into the PLL to generate the altered phase error response.
 15. Thesystem of claim 13, wherein the baseline phase error response isaccumulated over a first period within a sample period and the alteredphase error response is accumulated over a second period within thesample period.
 16. The system of claim 15, wherein the sample period isidentified as potentially indicative of the islanding condition when thethreshold is satisfied.
 17. The system of claim 16, wherein the phaseerror signature monitor (i) declares the islanding condition when afirst number of sample periods within a consecutive second number ofsample periods are identified as potentially indicative of the islandingcondition and (ii) inhibits power production in a corresponding inverterwhen the islanding condition is declared.